Epithelial tissues cover surfaces that are exposed to the environment, line internal passages and chambers, and form glands. They are found in the skin, lining the respiratory, reproductive, digestive, and uri-nary tracts. They also line the inner walls of the blood vessels and heart. Epithelia are found lining the vari-ous body cavities, such as the cerebral, spinal, peri-cardial, pleural, and peritoneal cavities.
As the major function of epithelia is to form a barrier, they are found in layers, with individual cells bound to adjacent cells, unlike other tissues that may be found scattered individually in the extracellular ma-terial. Cells may be bound to each other by fusion of cell membranes to form tight junctions. Tight junc-tions prevent movement of water and other sub-stances between the cells. In some epithelia, the bind-ing between the cells may be in the form of gapjunctions. Gap junctions have small passages that al-low movement of substances between adjacent cells. Other cells, such as those in the skin, are bound to-gether by desmosomes. These connections are strong and help to maintain the cell layers in sheets.
One surface of epithelial cells is exposed to the ex-ternal surface, such as the atmosphere or passage they line (lumen). This surface is the apical surface. The other surface faces the inside of the body and is known as the basal surface. The basal surface of the epithelia is attached to a thin, fibrous membrane known as the basement membrane.
As the cells of the epithelia are closely packed, they do not have blood vessels supplying them. Instead, they rely on nutrients brought by diffusion from ad-jacent blood vessels. The cells closer to the lumen may obtain nutrients by diffusion from the lumen. Being exposed to the environment, epithelial cells are constantly being damaged and lost; however, the stem cells located in the epithelia multiply rapidly and replace these cells constantly.
The epithelia located in areas of absorption or secre-tion, are modified to increase the surface area for this function. The modification is in the form of microvilli. Such epithelia are found in the digestive and urinary tract. Certain epithelia have cilia, which enables them to move secretions and other fluid over the surface. Cil-iated epithelia are found in the respiratory tract.
As previously mentioned, a major function of the ep-ithelia is to form a barrier and protect the body from dehydration, injury, and destruction by chemicals and foreign agents. Because the epithelia are selec-tively permeable to substances, they control the entry of substances into the body.
Almost all epithelia have a good nerve supply, which enables them to sense changes in the environ-ment and convey that information to the brain for suitable action. Some epithelia have a secretory func-tion and form the glandular epithelium.
As epithelia have common features as mentioned above, they are subtly modified to suit specific func-tions. Epithelia have been classified in accordance with the modifications in numbers of layers and with the shape of cell.
According to the number of layers, they are classi-fied as simple epithelium (one layer) or stratifiedepithelium (multilayered). According to cell shape,epithelia are classified as squamous, cuboidal, tran-sitional, and columnar.